A technique that is now widely used for insulating wires, and for producing coatings for other purposes, entails the exposure of a grounded workpiece to a cloud of electrostatically charged fusible particles, thereby causing the particles to deposit thereupon for subsequent integration. A well-recognized problem associated with the foregoing technique concerns the achievement of a uniform build upon the workpiece. When the apparatus employed is a fluidized bed unit the problem is most significant from the standpoint of achieving top-to-bottom uniformity, the lower surfaces of the wire tending to achieve a heavier build than the upper surfaces, primarily because they are closest to the source of the particle cloud.
The prior art has recognized these characteristics of electrostatic fluidized bed coating, and has proposed various solutions to the inherent problems. One effective approach is described and claimed in U.S. Pat. Nos. 4,297,386 and 4,330,567, to Gillette, wherein the characteristics of the particle cloud are controlled by electrical means. It is also common practice to mask the workpiece to control build, by interposing a physical barrier between it and the cloud, such as may be done by passing a wire to be coated through a tubular member, the extension of which into the coating chamber may be altered to vary the effective length of the workpiece exposed; this method is described, for example, in Beebe et al U.S. Pat. No. 3,396,699. Although the tubes utilized therein create a condition of either full exposure or full masking of the enclosed length of the workpiece, means for masking only a portion of the periphery is also known, as is disclosed in U.S. Pat. No. 4,011,832, to Westervelt, et al.
In any event, the configuration of the build control means utilized, as well as the effective distance over which it influences the deposit on the workpiece, will have a very significant effect upon the nature of the coating produced. This is especially so if only a portion of the workpiece periphery is to be masked by the control member, or if close control is to be achieved, in which case the dimensions and configuration of the barrier must normally be made to correspond substantially to the dimensions and configuration of the article being coated. It is necessary, moreover, that any commercially practicable electrostatic powder coating system be suited for use with a variety of workpieces, thus normally making a single build control member inadequate for optimal results.
Accordingly, it is a primary object of the present invention to provide a novel electrostatic powder coating system, which is capable of producing a controlled build of powder upon any of a variety of continuous length workpieces which differ from one another in dimensions and/or configurations.
It is a more specific object of the invention to provide such a system wherein the distance over which the workpiece is exposed to the powder cloud can readily be varied, thereby enabling facile alteration of the build.
It is also an object of the invention to provide a system having the foregoing features and advantages, in which the effective length and configuration of the build control member can be altered remotely and/or automatically, in response either to operator control or to a signal generated and fed back within the system.
Additional objects of the invention are to provide a novel method by which such build control is provided for the electrostatic powder coating of a workpiece, and to provide novel apparatus by which such control is achieved.